The present invention relates to the jacketing of steel pipes by means of extruding a hose-like or tube-like thermoplastic foil onto such pipes.
The jacketing of steel in a manner using the basic principles of the field of art to which the invention pertains is, for example, carried out by means of an extruder head having one or more annular nozzles. A low-pressure chamber is provided at the head and particularly around the entrance for the pipe to be jacketed, into the head. This axial low-pressure or vacuum chamber is also provided with a sealing sleeve, tightly engaging the surface of the pipe. The purpose of this vacuum chamber is to generate a low pressure between the foil-hose as emerging from the extruder nozzle and the outer surface of the pipe so that the foil-hose is forced tightly against the pipe's surface, under exclusion of air bubbles. The pressure is about 500 mm water column. It is apparent that this low-pressure chamber must be sealed against the outer atmosphere, bearing in mind that the pipe must also pass through and must transit from atmospheric pressure to the reduced pressure chamber. The sealing is provided particularly by a rubber sleeve which tightly fits around the passing pipe.
The known construction is disadvantaged by the fact that the pipes are usually preheated to about 200.degree. C. before entering the extrusion nozzle. As a consequence, the rubber sleeves leave smear marks on the pipe. These marks diminish the adhesion of the hose.